The present invention relates generally to the field of display systems for use in aircraft, land vehicles, and the like, and more particularly to a head-up/head-down display instrument utilizing cholesteric liquid crystal elements.
Many of todays more advanced aircraft, as well as many other advanced vehicles include a cathode ray tube-based head-up display ("HUD") and head-down display ("HDD") as cockpit instruments. Until recently, these were necessarily two separate units. There are now systems that combine the HUD and HDD in a single unit by using a single cathode ray tube ("CRT") that alternatively displays HUD and HDD symbology. When the CRT displays the HUD image, the optical system directs the image to the HUD screen and blocks it from the HDD screen. When the CRT displays the HDD image, the optical system directs the image to the HDD screen and blocks it from the HUD screen. The system switches back and forth between HUD and HDD images so quickly that the pilot or operator perceives both displays to be on simultaneously and continuously.
With reference to FIG. 1a, a typical prior art HUD/HDD system, designated generally with the reference numeral 10, is shown including a CRT 12, a vertical linear polarizer 14, a LCD 16, beam splitter 18, horizontal linear polarizer 20, vertical linear polarizer 22, and combiner 24. CRT 12 produces an image which is given a vertical polarization by vertical linear polarizer 14. The image is then incident upon LCD 16. LCD 16 is a controllable bire-fringent device (or, more commonly, a liquid crystal shutter--hereinafter "controllable LCD") that can be placed in one of two states, depending upon a control voltage level applied. Applying a low voltage to the controllable LCD 16 rotates the plane of polarization of transmitted light by 90.degree.; applying a higher voltage place the controllable LCD 16 in a state that permits it to transmit light with no change in polarization. Since a vertical linear polarizer can substantially block horizontally polarized light (and vice-versa), by selectively arranging horizontal and/or vertical linear polarizers in the image path, the image may be selectively transmitted or blocked in an HUD or HDD path by controlling the linear polarization of the image transmitted by the controllable LCD 16. Thus, if the image passed by the controllable LCD 16 is vertically polarized, that portion of the image passed by the beam splitter 18 will be blocked by the horizontal polarizer 20 while the reflected (by beam splitter 18) portion will be passed by the vertical linear polarizer 22 to the combiner 24 for viewing. Conversely, electrically switching the controllable LCD 16 to a high voltage state changes the vertically linear polarized image to a horizontally linear polarized image that is blocked by the vertical linear polarizer 22, and passed (for viewing) by the horizontal linear polarizer 20. An example of such an arrangement is summarized in FIG. 1b, corresponding to the embodiment shown in FIG. 1a.
Not shown in FIG. 1a for reasons of clarity, but evident to those skilled in this art, is the timing and control circuitry, and corrections to certain of the elements, for controlling generating of the HUD and HDD images, for controlling the controllable LCD 16, and for synchronizing that generation and control as appropriate.
Such prior art systems are subject to light leakage through the polarizers. That is, for example, horizontal linear polarizer 20 does not fully block the HDD path in the HUD mode, nor does vertical linear polarizer 22 fully block the HUD path in the HDD mode. Therefore, it is common in the prior art to "stack" polarizers to reduce light leakage. The common method is referred to as "double-extinguishing," and the more effective implementations of this method use at least three polarizers in the HUD or HDD image path.
Although light leakage is reduced, with some increase in the contrast of the image as against any background passing through combiner 24, there is a significant degredation in light transmission. Consider: If the polarizers are chosen to have a normal transmission of 38% then the maximum light transmission of the system would be in the neighborhood of 5.5%. Increasing the number of polarizers decreases the light leakage but correspondingly decreases transmission of the system.
The present invention overcomes the low contrast/transmission problems inherent in prior art single-source HUD/HDD systems. As discussed below, the present invention finds application in both monochrome and color systems.